1. Technical Field
The present invention relates to the delivery of electronic mail (E-mail) messages in a wired or wireless communications system and, more specifically, to the delivery of E-mail messages of unrestricted length to a fixed or mobile subscriber who can learn the contents of those messages without being distracted from performing other activities.
2. Related Prior Art
The prior art includes cellular radio systems which have been operating in the United States and Europe for the last two decades. Cellular telephone service operates much like the fixed, wireline telephone service in homes and offices, except that radio frequencies rather than telephone wires are used to connect telephone calls to and from the mobile subscribers. Each mobile subscriber is assigned a private (10 digit) directory telephone number and is usually billed based on the amount of "airtime" he or she spends talking on the cellular telephone each month. Many of the service features available to landline telephone users (e.g., call waiting, call forwarding, three-way calling, etc.) are also generally available to mobile subscribers. In each market area, mobile subscribers usually have the freedom to subscribe to service from at least two systems. The local system from which service is subscribed is called the "home" system. When travelling outside the home system, a mobile subscriber may be able to obtain service in a distant system if there is a "roaming" agreement between the operators of the home and "visited" systems.
The architecture for a typical cellular radio system is shown in FIG. 1. A geographical area (e.g., a metropolitan area) is divided into several smaller, contiguous radio coverage areas, called "cells", such as cells C1-C10. The cells C1-C10 are served by a corresponding group of fixed radio stations, called "base stations", B1-B10, each of which includes a plurality of radio frequency (RF) channel units (transceivers) that operate on a subset of the RF channels assigned to the system, as well known in the art. The RF channels allocated to any given cell may be reallocated to a distant cell in accordance with a frequency reuse plan as is also well known in the art. In each cell, at least one RF channel, called the "control" or "paging/access" channel, is used to carry control or supervisory messages. The other RF channels are used to carry voice conversations and thus are called the "voice" or "speech" channels. The cellular telephone users (mobile subscribers) in the cells C1-C10 are provided with portable (hand-held), transportable (hand-carried) or mobile (car-mounted) telephone units, collectively referred to as "mobile stations", such as mobile stations M1-M5, each of which communicates with a nearby base station. Each of the mobile stations M1-M5 includes a microphone, a loudspeaker, a controller (microprocessor) and a transceiver, as well known in the art. The transceiver in each mobile station may tune to any of the RF channels specified in the system (whereas each of the transceivers in the base stations B1-B10 usually operates on only one of the different RF channels used in the corresponding cell).
With continuing reference to FIG. 1, the base stations B1-B10 are connected to and controlled by a mobile telephone switching office (MTSO) 20. The MTSO 20, in turn, is connected to a central office (not specifically shown in FIG. 1) in the landline (wireline) public switched telephone network (PSTN) 22, or to a similar facility such as an integrated services digital network (ISDN). The MTSO 20 switches calls between wireline and mobile subscribers, controls signalling and assignment of voice channels to the mobile stations M1-M5, compiles billing statistics, stores subscriber service profiles, and provides for the operation, maintenance and testing of the system. An important function of the MTSO 20 is to perform a "handoff" of a call from one base station to another base station B1-B10 as one of the mobile stations M1-M5 moves between cells C1-C10. The MTSO 20 monitors the quality of the voice channel in the old cell and the availability of voice channels in the new cell. When the channel quality falls below a predetermined level (e.g, as the user travels away from the old base station towards the perimeter of the old cell), the MTSO 20 selects an available voice channel in the new cell and then orders the old base station to send to the mobile station on the current voice channel in the old cell a handoff message which informs the mobile station to tune to the selected voice channel in the new cell.
The original cellular radio systems, as described generally above, used analog transmission methods, specifically frequency modulation (FM), and duplex (two-way) RF channels in accordance with the Advanced Mobile Phone Service (AMPS) standard. This original AMPS (analog) architecture formed the basis for an industry standard sponsored by the Electronics Industries Association (EIA) and the Telecommunications Industry Association (TIA), and known as EIA/TIA-553. In the middle to late 1980s, however, the cellular industry both in the United States and in other parts of the world began migrating from analog to digital technology, motivated in large part by the need to address the steady growth in the subscriber population and the increasing demand on system capacity. The industry thus developed a number of air interface standards which use digital voice encoding (analog-to-digital conversion and voice compression) and advanced digital radio techniques, such as time division multiple access (TDMA) or code division multiple access (CDMA), to multiply the number of voice circuits (conversations) per RF channel (i.e., to increase capacity).
In Europe and Japan, the GSM and PDC standards, respectively, both of which use TDMA, have been widely implemented. In the United States, the EIA/TIA has developed a number of digital standards, including IS-54 (TDMA) and IS-95 (CDMA), both of which are "dual mode" standards in that they support the use of the original AMPS analog voice channels (AVCHs) and analog control channel (ACCH), in addition to newer digital traffic channels (DTCHs) defined within the existing AMPS framework, so as to ease the transition from analog to digital and to allow the continued use of existing analog mobile stations. The dual-mode IS-54 standard, in particular, has become known as the digital AMPS (D-AMPS) standard. More recently, the EIA/TIA has developed a new specification for D-AMPS, which includes a digital control channel (DCCH) suitable for supporting data services and extended mobile station battery life. This new specification, which builds on the IS-54B standard (the current revision of IS-54), is known as IS-136.
FIG. 2 shows the forward RF channel (base station to mobile station) as generally specified in the IS-136 standard. Referring to the upper part of FIG. 2, each RF channel comprises a series of repeating time slots which are grouped into 40 ms frames carrying from three to six DTCHs depending on the source rate of the speech coder used for each DTCH. According to IS-136, the speech coder for any DTCH can operate at either "full-rate" or "half-rate". A full-rate DTCH occupies two slots per frame (channel "A", "B" or "C" in FIG. 2) while a half-rate DTCH occupies one slot per frame (not shown for the sake of simplicity). At call set-up or handoff, a dual-mode mobile station will be assigned preferably to a DTCH (full-rate or half-rate) and, if none is available, it can be assigned to an AVCH. An analog-only mobile station, however, can be assigned only to an AVCH.
In IS-136, the DCCH is defined similarly to the DTCH (i.e., the DCCH and DTCH use the same TDMA frame format and slot size, and can share the same RF channel). Thus, a half-rate DCCH would occupy one slot while a full-rate DCCH would occupy two slots out of the six slots in each 40 ms frame. The DCCH slots are mapped to different logical channels which are organized into a series of superframes. The lower part of FIG. 2 shows the superframe structure of a full-rate DCCH (in this example, the DCCH is defined over channel "A" in the TDMA frame). A superframe is defined in IS-136 as the collection of 32 consecutive time slots (640 ms) for a full-rate DCCH (16 slots for a half-rate DCCH). The logical channels specified in IS-136 include a broadcast control channel (BCCH) for carrying system-related information which is broadcast to all mobile stations, and a short message service, paging and access response channel (SPACH) for carrying information which is sent to specific mobile stations.
As shown in FIG. 2, the BCCH is divided into logical subchannels each of which is assigned an integer number of DCCH slots. The BCCH subchannels include a fast BCCH (F-BCCH), an extended BCCH (E-BCCH) and a point-to-multipoint short message service BCCH (S-BCCH). The F-BCCH is used to broadcast DCCH structure parameters and other information required for accessing the system (the first slot in a superframe is always assigned to the F-BCCH). The E-BCCH, which may span several superframes, is used to broadcast information that is not as time-critical (for the operation of the mobile stations) as the information in the F-BCCH. The S-BCCH is used for the broadcast short message service (SMS), which can deliver alphanumeric messages of common interest to all mobile stations (e.g., traffic reports).
The SPACH is also divided into logical subchannels each of which is assigned a given number of time slots on a fully dynamic basis (and, thus, these subchannels are not explicitly shown in FIG. 2). The SPACH subchannels include a point-to-point short message service channel (SMSCH), a paging channel (PCH) and an access response channel (ARCH). The SMSCH is used for carrying alphanumeric messages of interest to a specific mobile station (e.g., stock quotations). The PCH is used for carrying paging messages to different mobile stations. The ARCH is used for responding to access requests from one of the mobile stations (e.g., by delivering a channel assignment message to that mobile station).
Among the uses slated for the SMSCH specified in IS-136 is the delivery of electronic mail (E-mail) messages to the mobile subscribers. However, there are two practical problems with the delivery of E-mail messages via the SMSCH (or the equivalent DCCH subchannel specified in other digital standards). First, messages sent on the SMSCH are displayed on a small liquid crystal display (LCD) screen (or a similar display screen) in the mobile station. This means that the user of the mobile station may be distracted from his other activities (e.g., driving, working, etc.) while reading and/or scrolling through a typical E-mail message which exceeds the display capacity of the LCD screen. In certain instances (e.g., if the user is operating a vehicle, heavy machinery or other equipment), such distractions may present a danger to the safety of the user and/or other persons.
Second, SMSCH messages are inherently limited in length to a certain number of characters (bytes). For example, according to IS-136, SMSCH messages must not be longer than 250 characters. Taking into consideration the signalling overhead requirements, the effective length of SMSCH messages is reduced even further to approximately 239 characters. Although it may be possible to use the EBCCH (or a similar DCCH subchannel) for sending E-mail messages of greater length than 239 characters, that would result in the division of the text of the E-mail message over two or more superframes at arbitrary points in the text. Such an arbitrary division (i.e., "chopping") would be likely to further complicate the task of reading the E-mail message at the mobile station, and to exacerbate the risk of the user becoming distracted while reading the message.
Therefore, there is a need for a new E-mail delivery system which does not restrict the length of an E-mail message to a mobile subscriber, and which will allow the mobile subscriber to learn the contents of the E-mail message without being distracted from performing other activities.